Automatic number plate recognition system

ABSTRACT

A handheld automatic number plate recognition system comprising a handheld personal computer and a camera, the camera being operable to capture images and the computer including a processor to identify a number plate from an image captured by the camera.

THIS INVENTION RELATES to an automatic number plate recognition systemand more particularly to a handheld automatic number plate recognitionsystem.

Automatic number plate recognition systems comprise a frame grabbernormally in the form of a video camera, charge coupled device or thelike in combination with a computer system which incorporates analgorithm to recognise and interpret the characters making up thelicence number on a number plate. Automatic number plate recognitionsystems typically use a powerful computing platform such as a Pentiumbased desk top computer. Video images are captured from video camerasoften working in the infrared spectrum. The cameras are fixedly mountedat the roadside, on motorway bridges, on gantries and/or on or neartraffic barriers and gates. These fixed installations are primarily usedfor obtaining and reporting details of passing traffic, for loggingstaff or visitors onto commercial premises and allowing or restrictingaccess to such premises by automatically opening barriers or the like.Traditional systems are based on standard or industrial personalcomputers and standard video cameras usually set to a high shutter speedof at least {fraction (1/500)}th sec., the resultant combination beingbulky, expensive, difficult to install, necessitating hardwiring andsufficiently permanent fixing to prevent tampering. Since theinstallations are usually open to the elements, they require routinechecking and maintenance.

The fixed installation systems cannot perform the full range offunctions since they are by their definition, fixed to a dedicated site.Whilst such a fixed installation may be useful, for example, to monitorthe ingress and egress of vehicles at a car park entrance or exit, thesystems are not able to be used by personnel to check which actualvehicles are present in a car park during an emergency situation.

It is an object of the present invention to seek to provide an automaticnumber plate recognition system which does not suffer from the abovementioned problems.

Accordingly, one aspect of the present invention provides a handheldautomatic number plate recognition system comprising a handheld personalcomputer and a camera, the camera being operable to capture images andthe computer including a processor to recognise a number plate from animage captured by the camera.

Advantageously, the computer is operable to power down the camera whennot in use.

Preferably, the processor includes means to recognise and identifyrectangular areas, each such recognised area comprising a possiblenumber plate to be recognised.

Conveniently, the processor sets a number plate threshold range outsideof which a captured image will not be processed.

Advantageously, the threshold range is between 1,000 and 10,000 pixels.

Preferably, the threshold range is between 1,800 and 5,000 pixels.

Conveniently, the computer has a memory in which is stored a pluralityof known number plates.

Advantageously, the system is portable.

Preferably, the camera is attached to the computer.

Conveniently, the computer includes a display operable to indicate arecognised number plate.

Advantageously, the computer includes a speaker and a synthesiser toenunciate the characters making up a recognised number plate.

Preferably, an infrared light source and battery pack for the infraredlight source is separately attached to the system.

Conveniently, the infrared light source comprises a cluster of LED lightsources.

Advantageously, any infrared cut filter provided in the camera isremoved.

Preferably, the camera is provided with an infrared band-pass filterallowing infrared light to pass into the camera.

Conveniently, the infrared band-pass filter allows infrared light havinga wavelength of between 800 and 900 nanometres to enter the camera.

Advantageously, a separate module is fitted to the system containing abattery and a modem to provide the system with a telecommunicationcapability.

Preferably, the system is operable to respond to a recognised numberplate matched with a stored number plate by transmitting data from thatplate to a remote computer for subsequent action.

Conveniently, the mode of telecommunication is an SMS text message.

In order that the present invention may be more readily understood,embodiments thereof will now be described, by way of example, withreference to the accompanying drawings, in which:

FIG. 1 is an automatic number plate recognition system embodying thepresent invention.

A preferred embodiment of the present invention is shown in FIG. 1 andcomprises a handheld personal computer 1 which in the present instancecomprises a Casio Cassopeia E115, E125 or an EG800 or equivalentsthereof having an expansion slot to accept a camera or frame grabber 2.The computer 1 has a processor, a memory, a plurality of user interfacekeys or buttons 3 and a touch sensitive display screen 4. With thecamera 2 attached to the hand held computer 1, the resultant unit meetsthe hardware requirements of an automatic number plate recognitionsystem.

The processor is operable to run automatic number plate recognitionsoftware which comprises an engine which runs under Windows CE or PocketWindows.

The automatic number plate recognition software processes imagescaptured by the camera. Each image is frozen and the software scans eachimage in the first instance to look for and identify any rectangularboundaries. In order to reduce the amount of processing required to bedone by the computer and to ensure that only readable number plates arepresented for processing, the software applies a minimum size of numberplate to the identified rectangular boundaries or shapes. Accordingly,the user of the system is able to set a threshold so that rectangularshapes comprising number plates below a certain size are ignored by thesoftware. It has been found that an optimum minimum size to ensure thata number plate is readable by the software is in the region of 1,800 to5000 pixels but possibly as low as 1,000 pixels and as high as 10,000pixels.

Having identified rectangular areas having a pixel size in the thresholdrange, the software copies the original image which is then analysed tosearch for the characters making up the number plate.

Each character within the rectangular area is given a confidence factorand a semantic check is carried out. For example, number plates fromcars of different nationalities have different formats, the letters andnumbers making up the number plate being arranged according to differentrules. For example, British number plates are at the present timedivided into three groupings comprising a letter prefix followed bythree numbers and a three letter suffix. The three letter suffix isspaced apart from the three numbers. A Dutch number plate comprisesthree groupings consisting of a two letter prefix, a two number groupand a two letter suffix, the groups being separated by dashes. Thesemantic check can be applied to the number plate either by assuming orexpecting that in a default mode all number plates will be in the formatof the country where the device is being used or to determine the likelynationality of a vehicle and to provide a better likelihood of successin distinguishing between confusingly similar letters and numbers suchas “B” and the numeral “8” on the basis of the location of the characterin the group of characters making up the number plate. For example, itis more likely that the first character in a British number plate willbe the letter “B” rather than the numeral “8”.

An overall confidence factor is then applied to the number plate.

The algorithms which apply the confidence factors and the overallconfidence factor work by matching character shapes to a predefined setof standard fonts looking for similarities therebetween and according aconfidence factor depending upon the number of similarities found. It isimportant to note that the operating software is not neural net based assuch techniques are only suitable in certain locations with certainpatterns of traffic and can be over-taught or “grand-mothered”.

The resolution of the camera 2 for use with the handheld computer 1 isapproximately half the resolution of a standard video grabber, being320×240 pixels as opposed to 640×288. A resolution at 320×288 isenvisaged for more powerful processors. Thus, the software is configuredto set a fairly narrow threshold range of between at least 1,000 pixelsand no more than 10,000 pixels, the optimum size range to be processedbeing between 1,800 and 5,000 pixels.

Owing to the slower processing speeds and battery constraints ofhandheld personal computers, the automatic number plate recognitionsystem embodying the present invention has been adapted to operatesignificantly differently from non-portable systems.

A preview mode can be selected when the camera is switched on so that alive video preview is shown on the display screen 4 allowing a user topoint the device accurately at a parked vehicle. When the user presses a“capture” button 3, the frame is grabbed and processing begun.

Once an image has been captured, only the region recognised as thenumber plate, the plate patch, is saved. The plate patch is processedand the number plate details are displayed on the screen and preferablyan audible tone is provided.

In a preferred embodiment the automatic number recognition system ispre-loaded with a list of known number plates.

The displayed number is looked up in a database held by the computer ofa stored list of “wanted” number plates and compared with all theentries and if a match is found, then a predetermined message isdisplayed and preferably a further warning tone is emitted. The databasecan be readily and regularly updated.

Upon a match being detected, further information concerning theidentified vehicle can be obtained by pressing a “details” button 3which displays information such as the make, model, drivers details andthe date of the last visit of the vehicle to the location. An “action”button 3 is also provided to display the letters and numbers of theidentified number plate in police call signs, for example “S 443 NNR”would be displayed as:

-   -   Sierra    -   Four    -   Four    -   Three    -   November    -   November    -   Romeo.

The data relating to the frame grabbing operation comprising the numberplate details, the time and date and the location, if known, are savedin the computer's memory for later uploading when synchronising thehandheld PC with a master PC.

In this simple mode of operation, the camera 2 is powered down aftereach frame is grabbed and the real time preview is also disabled so asto save power. Indeed, in another embodiment of the present invention,the live preview mode can be dispensed with by confident users who areable simply to point the system at a vehicle and press the “capture”button to grab an appropriate frame.

Other modes of operation which are possible comprise the ability to grabmultiple frames. The ability to grab multiple frames gives higheraccuracy of number plate recognition. Typically, the software would grabseveral images, in the region of 3 to 5 grabs, and analyse all of them.Thus, several distinct attempts are made to analyse the number plate ofa vehicle and arrive at a consensus decision to report the recognisednumber plate and make this information available for display, storage,or activation of a device by the computer.

It has been noted that the camera 2 provides a substantial drain on thepower resources of the handheld personal computer 1 and is accordinglydisabled whenever possible to prolong battery life. Battery life is inthe region of four hours but further battery packs can be providedhaving four, seven or thirteen hours respectively. A particularadvantage of the Casio handheld PC's is that the batteries can beunclipped and swapped in the field without loss of data.

The automatic number plate recognition software can also be expanded toinclude a voice synthesis module to speak the recognised number plateout loud using call signs. This feature is extremely useful in brightlight conditions when the screen is hard to read. This digital moduledoes, however, use additional battery power. So as to give someshielding from ambient light which can insert a condition to make thescreen of the hand held personal computer difficult to read, a housingfor the personal computer and camera is provided incorporating ashielding cowling.

Automobile number plates have a retro-reflective backing, the charactersmaking up the number plate being non-reflective. The retro-reflectivebacking reflects in the region of 85% of the incident light in theinfrared spectrum making the number plate extremely visible whenilluminated by an infrared light source.

In one embodiment of the present invention, the camera 2 is providedwith a camera lens 5 and a plurality of infrared light sources 6 eitherside of the lens. Any standard infrared filter provided on the camera ispreferably removed to facilitate camera operation in the infraredspectrum. It is important that the infrared light sources 6 are locatedadjacent the lens 5 because the retro-reflective nature of the backingrequires the incident light and the reflective light paths to beseparated by no greater than in the region of 5°. Greater separation ofthe light paths dramatically reduces the percentage of reflected lightthereby reducing the performance of the system. Not only does infraredoperation of the device allow the device to be used in low light levelapplications but also greatly serves to improve recognition of numberplates in cluttered environments. Also, in instances where therectangular number plate is not clearly bordered by a black outline orthe like, the infrared illumination of the number plate area serves toclearly distinguish this rectangular portion from its surroundings.

It is also envisaged that a small cluster of light emitting diodes canbe provided in a package which is strapped or clipped to the computer 1or frame grabber/camera 2, the package containing rechargeable batteriesfor powering the LED cluster. The cluster of LED light sources serve toilluminate number plates at a distance of 2 or 3 metres and allow highquality reading in low light conditions or night time use.

It is important to ensure the clearest reading and to maximise powerefficiency that the number of frame grabs required to capture accuratelya recognisable image is minimised. To this end, any infrared cut filterprovided in the camera 2 is removed. This allows more infrared light toenter the camera but as a consequence distorts the colours of thecaptured image but this is not problematic for the recognition softwarewhich is able to concentrate on the clearly delineated reflectedinfrared image from the plate to be recognised.

Preferably, any infrared cut filter is replaced with an infraredband-pass filter which permits infrared light in the bandwidth range800-900 nanometres to enter the camera. This further improves theoperational performance of the recognition software minimising thenumber of frame grabs required to capture a recognisable number. Thesesteps also serve to maximise battery life by making relevant imagecapturing more efficient.

It is also envisaged that a clip-on module can be fitted underneath theunit containing a larger battery and a small GSM modem which gives theunit a dialling and messaging capability. The software is structured torespond to a number plate hit (i.e. the recognition of a “wanted” numberplate held on the computer database) by transmitting data from thatplate (e.g. the plate details, location, time and date) to a remote hostcomputer for subsequent action. The mode of communication may be as ashort message service (SMS) text message to a mobile phone, to a pageror directly to the remote computer.

The provision of the GSM modem allows a user to request and download anupdate from the remote computer of the latest number plate database (hitlist) which is transmitted to the unit over the GSM network and used toupdate the stored database. This removes the need to return to base togather the latest data on, for example, the latest stolen or suspectcars.

It is also envisaged that the camera and hand held personal computer beconfigured within a single housing, preferably ruggedised.

Preferably, the infrared light sources 6 should be controllable by thehand held personal computer so as to be pulsed to illuminate the subjectonly whilst the camera is in operation thereby saving battery power.

It is preferred that the hand held automatic number plate recognitionsystem is operable to output just a single number plate rather thanoffering a plurality of close alternatives.

It is also envisaged that a more powerful battery belt may be providedwhich connects to the hand held personal computer by means of a cable.

In the present specification “comprises” means “includes or consists of”and “comprising” means “including or consisting of”.

The features disclosed in the foregoing description, or the followingclaims, or the accompanying drawings, expressed in their specific formsor in terms of a means for performing the disclosed function, or amethod or process for attaining the disclosed result, as appropriate,may, separately, or in any combination of such features, be utilised forrealising the invention in diverse forms thereof.

1. A handheld automatic number plate recognition system comprising ahandheld personal computer and a camera, the camera being operable tocapture images and the computer comprising a processor to identify anumber plate from an image captured by the camera, wherein the processorcomprises means to identify rectangular areas, each such identified areacomprising a possible number plate to be interpreted.
 2. A systemaccording to claim 1, wherein the computer is operable to interpret theimage of the plate and output a string of recognized characters inresponse thereto.
 3. A system according to claim 1 or 2, wherein thecomputer is operable to power down the camera when not in use.
 4. Asystem according to any preceding claim, wherein the processor includesmeans to identify rectangular areas, each such identified areacomprising a possible number plate to be interpreted.
 5. A systemaccording to any preceding claim, wherein the processor sets a platesize threshold range outside of which a captured image will not beprocessed.
 6. A system according to claim 5, wherein the threshold rangeis between 1,000 and 10,000 pixels.
 7. A system according to claim 6,wherein the threshold range is between 1,800 and 5,000 pixels.
 8. Asystem according to any preceding claim, wherein the computer has amemory in which is stored a plurality of known number plates.
 9. Asystem according to any preceding claim, wherein the system is portable.10. A system according to any preceding claim, wherein the camera isattached to the computer.
 11. A system according to any preceding claim,wherein a single housing holds the camera and the computer.
 12. A systemaccording to any preceding claim, wherein the computer includes adisplay operable to indicate a recognized number plate.
 13. A systemaccording to any preceding claim, wherein the computer includes aspeaker and a synthesizer to enunciate the characters making up arecognized number plate.
 14. A system according to any preceding claim,wherein the camera is operable in the infrared spectrum and the systemhas at least one infrared source.
 15. A system according to claim 14,wherein the or each infrared source is located adjacent the camera suchthat a path of incident light from the infrared source is within 5° of areturn path of light reflected from a subject. 16-25. (Canceled).